Amazing!!!
The game play, while not entirely unique, is most similar to other non-linear side scrolling titles like Castlevania and Metroid. You get new abilities, forms, spells, etc that allow you to access new areas. And speaking of areas, they are huge! Sometimes it is a bit overwhelming how much area you have available to you. This game has a huge amount of secrets to discover, and has a lot of emphasis on exploration (in comparison to Castlevania and Metroid.) The downside of this is that sometimes it takes a while to travel around, but you can ride sea horses and turtles (like warps) to travel faster.
Near the beginning of the game, stuff is pretty slow. The environments are friendly, and there aren't usually multiple hostile creatures attacking you at once. This heats up very quickly, and eventually you'll find yourself riding some ocean current through hordes of enemies. I do admit, sometimes this game is just overwhelming and is a bit on the difficult side.
Character progression is mainly done with spells and cooking. Spells are learned at key areas in the game, that let you do things like pick up boulders and turn into new forms (personal favorite is the energy form which lets you toast enemies.) You also gather ingredients from dead enemies and the environment. With these, you can cook up food that has a plethora of effects, from stat buffing, healing, etc. The catch is you have to find the recipe first, otherwise any mixture that is unknown will automatically become sea-loaf (kinda like meatloaf that gives +.1 hp.) You can also collect treasures for your house that have different effects, most are just aesthetic though (some are wearable costumes).
The graphics are, simply put, very nice. The technology may be old, but it is one of the best looking 2d games I have seen. Everything is hand drawn, and looks great. I couldn't imagine this game looking any better. The environments seem alive, schools of fish will swim around, and seaweed drifts around in the ocean currents. When you dart through the water, there is a nice wavy 3d-distortion effect.
The music, also is really catchy. Sound effects are all very suitable as well. They don't jump out at you, but they aren't bad. The voice acting is very good as well. Which leads me to the next topic...
The story is very good. It takes a while to pick up, but the narrative and point of view (told through the main character's eyes) is mysterious and gives a sort of emptiness to the game. I mean this in a good way, and it is definitely by design.
So, in conclusion, if you have any interest in playing a side-scrolling non-linear adventure game, play this one. It easily surpasses the quality of big-name publisher releases like Castlevania and Metroid, and has an art style and uniqueness to it that you can rarely find in games these days.